Police said 23-year-old Najai Turpin shot himself in the head at 4 a.m. Monday while sitting with his girlfriend in a parked car outside the West Philadelphia gym where he trained. Investigators were unsure why he took his life.

An NBC spokeswoman said the producers, cast and crew of “The Contender” were shocked and saddened by Turpin’s death. The program, scheduled to debut March 7, will go on as planned.

Produced by reality TV mogul Mark Burnett, “The Contender” will follow the lives of 16 boxers competing against each other for a chance at a million-dollar purse.

The episodes involving Turpin had already been taped.

Percy “Buster” Custus, a trainer who had worked with him since Turpin was 12, said the boxer had enjoyed his experiences with the show but seemed troubled in recent weeks. He said Turpin abruptly left a training camp in the Poconos and returned home to Philadelphia, saying he missed his family.

“None of us really know what brought this about,” Custus said Tuesday. “You just want to see the boys come out of the neighborhood. From the time they’re young kids, you really want to see them make it. And he was right there.”

Turpin had a 13-1 record and had won a city Recreation Department title before he was picked for “The Contender.”

A biography on the show’s Web site called him an “extremely soft-spoken” but focused fighter who worked two day jobs to support his family. He had a 2-year-old daughter.

“You would never know he was a fighter,” Custus said. “He was a tough guy in the ring. He was a vicious fighter. But outside the ring he was a different man.”

Police said they didn’t know where Turpin got the gun. He was not licensed to carry a handgun and the weapon was not registered to him.

NBC released a statement from Burnett in which he called Turpin a “great fighter with tremendous heart and courage.”

“The episode in which he was most depicted will stand as a wonderful testament to who he was. It will not be changed,” Burnett wrote.

A tribute to Turpin will be added to the show. Viewers will also be offered a chance to donate money to a trust fund set up to support Turpin’s child.

NBC spokeswoman Rebecca Marks declined to say how Turpin had fared in the show, which is to conclude with a live championship bout between two finalists in May. She said, however, that the show will not need to be overhauled because of Turpin’s death.

Every contestant was being paid $1,500 per week to stay in training pending the finale.

Turpin worked out at the James Shuler Memorial Gym, a haven for serious fighters from a rough and impoverished neighborhood.

Tybius Flowers, another boxer at the gym, was murdered last year shortly before he was to appear as a key witness in a murder trial.